The 12 authorized purchasers are the only firms that are allowed to purchase finished bullion coins from the mint. The only direct to public sales are for uncirculated and proof versions of coins. The authorized purchasers are only allowed a limited markup and have pretty good reputations.
Sunshine and others manufacture precisely sized and finished blanks or planchetts directly to the mint to be used in the coin presses.
All of the bullion that the US Mint uses must be domestically produced in the last year. The mint is actually one of the few parts of the govt that turn a profit. They could easily double that profit if they privatized their collectible coin operations to be more responsive to the public.